1. Field of the Invention
The present invention relates to a photosensitive resin composition and a color filter provided with a protective layer formed of the photosensitive resin composition, and more particularly to a color filter which can realize color liquid-crystal displays having excellent display quality and high reliability.
2. Background Art
In recent years, color liquid-crystal displays have drawn attention as flat displays. They generally have a structure comprising: a color filter; a counter electrode substrate facing the color filter; and a liquid-crystal layer provided in a gap (1 to 10 xcexcm) between the color filter and the counter electrode substrate. The structure of the color filter is such that a colored layer of a black matrix and a plurality of colors (generally three primary colors, red (R), green (G), and blue (B)) or, in recent years, a color hologram is provided on a transparent substrate and a protective layer and a transparent electrode are stacked thereon in that order. These color liquid-crystal displays are constructed so that a color image is obtained by controlling the light transmission of the liquid-crystal layer in its portions corresponding to pixels of respective colored layers R, G, and B or a color hologram.
In the color filter, the protective layer, when the color filter comprises the colored layer, functions to protect and flatten the color filter. In general, color liquid-crystal displays have a problem that the presence of uneven gap attributable to the waviness of the surface of the transparent substrate in the color filter, uneven gap among R, G, and B pixels, or uneven gap within pixels of R, G, and B lowers the flatness of the transparent electrode. This causes uneven colors and uneven contrast, leading to lowered image quality. In particular, in color liquid crystal displays of STN (super-twisted nematic) system, the flatness greatly affects the image quality. This renders flattening by the protective layer very important.
In use, the color filter is adhered to the counter electrode. The assembly is then tested for display quality. In consideration of reutilizability of the color filter when the assembly is judged to be unacceptable in the display quality test, the protective layer is preferably provided only in specific regions so as to cover the colored layers on the transparent substrate. To this end, the protective layer has been formed using a photocurable resin which permits portions to be cured to be easily limited through a mask.
In the formation of the protective layer using the conventional resin, an organic solvent is used in the development after exposure. This is troublesome in handleability and wastewater treatment and further lacks in profitability and stability. In order to solve this problem, a photocurable resin has been developed which, by virtue of the introduction of an acidic group into the photocurable resin, permits alkali development after the exposure. An example of the alkali-soluble photocurable resin known in the art is o-cresol novolak epoxyacrylate having a weight average molecular weight of about 2,000. This has as monomer units carboxylic acid groups governing alkali solubility and acryloyl groups governing curability. Therefore, the proportions of the carboxylic acid group and the acryloyl group in the resin is constant.
For this reason, introduction of a large amount of an acryloyl-modified, low-molecular weight polyfunctional alcohol, for example, a hexafunctional alcohol, dipentaerythritol hexaacrylate (DPHA), is considered effective for improving the density of the radical polymerizable group in the photosensitive resin composition. This method, however, results in excessively lowered viscosity of the photosensitive resin composition, posing a problem that the coatability is deteriorated. Further, this involves an additional problem that, when the photosensitive resin composition is coated for the formation of the protective layer in the color filter, cracks are created in the coating due to remarkable shrinkage upon curing and the adhesion of the protective layer is poor.
Typical conventional methods for introducing radically reactive groups, such as (meth)acryloyl groups, include, for example, regarding the preparation of urethane acrylate, a method which comprises reacting a diol compound with a diisocyanate in an excessive amount to give a product with the isocyanate group being left at the ends and reacting this isocyanate group with 2-hydroxyethyl methacrylate to introduce a radically polymerizable group, such as a methacryloyl group, into the ends of the reaction product. In this method, in principle, the methacryloyl group is introduced into only both ends. Incorporation of a compound having two or more (meth)acryloyl groups as a part of the system followed by radical polymerization is also considered. According to this method, however, the content of the radically reactive group cannot be regulated. Further, this method involves a problem of gelation and the like.
Thus, for the photocurable resins, it is difficult to regulate alkali-soluble groups, such as carboxyl groups, and radically polymerizable groups, such as (meth)acryloyl groups, for attaining desired curability, alkali solubility and the like. Further, photosensitive resin compositions having not only excellent curability and alkali solubility but also excellent coating properties have not been proposed in the art.
Accordingly, it is an object of the present invention to provide a photosensitive resin composition which enables the content of alkali-soluble groups and radically polymerizable groups, such as (meth)acryloyl groups, to be regulated and at the same time possesses excellent curability, alkali solubility, and coating properties, and a color filter which can be produced by a simple process, has excellent flatness, and can realize highly reliable color liquid-crystal displays having excellent display quality.
According to one aspect of the present invention, there is provided a photosensitive resin composition comprising: a copolymer resin; a bifunctional or higher polyfunctional photopolymerizable acrylate monomer; an epoxy resin; and an initiator,
said copolymer resin comprising 5 to 55% by mole of constituent units represented by formula (1) and 5 to 95% by mole of constituent units represented by formula (2), the constituent units represented by formula (1) having been partially reacted, through carboxyl groups thereof, with a (meth)acryloylalkyl isocyanate compound, the constituent units represented by formula (2) having been partially reacted, through hydroxyl groups thereof, with a (meth)acryloylalkyl isocyanate compound, said copolymer resin containing 5 to 95% by mole of (meth)acryloyl groups and having an acid value of 5 to 400 mg KOH/g and a weight average molecular weight of 10,000 to 1,000,000 as determined using polystyrene as a standard: 
wherein R represents hydrogen or an alkyl group having 1 to 5 carbon atoms and R1 represents an alkylene group having 2 to 4 carbon atoms.
According to another aspect of the present invention, there is provided a color filter comprising: a transparent substrate; a colored layer provided on the transparent substrate; and a protective layer provided so as to cover the colored layer,
said protective layer having been formed by coating a photosensitive resin composition, exposing the coating, developing the exposed coating with an alkali, and heating the developed coating,
said photosensitive resin composition comprising: a copolymer resin; a bifunctional or higher polyfunctional photopolymerizable acrylate monomer; an epoxy resin; and an initiator, said copolymer resin comprising 5 to 55% by mole of constituent units represented by formula (1) and 5 to 95% by mole of constituent units represented by formula (2), the constituent units represented by formula (1) having been partially reacted, through carboxyl groups thereof, with a (meth)acryloylalkyl isocyanate compound, the constituent units represented by formula (2) having been partially reacted, through hydroxyl groups thereof, with a (meth)acryloylalkyl isocyanate compound, said copolymer resin containing 5 to 95% by mole of (meth) acryloyl groups and having an acid value of 5 to 400 mg KOH/g and a weight average molecular weight of 10,000 to 1,000,000 as determined using polystyrene as a standard.
According to a preferred embodiment of the present invention, the copolymer resin in the photosensitive resin composition and the color filter further comprises 0 to 75% by mole of constituent units represented by formula (3) and 0 to 75% by mole of constituent units represented by formula (4): 
wherein R represents hydrogen or an alkyl group having 1 to 5 carbon atoms, R2 represents an aromatic carbon ring, and R3 represents an alkyl group or an aralkyl group.
The photosensitive resin composition of the present invention, by virtue of use of a copolymer resin having controlled alkali solubility, and curability, possesses excellent tackiness, leveling, alkali resistance, adhesion, developability, edge shape, surface appearance, and dynamic hardness. Use of this photosensitive resin composition in the formation of a protective layer can realize a color filter which can be easily produced and has excellent flatness and display quality and high reliability.